arm: mvebu: Add armada-xp-gp-u-boot.dtsi for U-Boot properties

Add some missing "u-boot,dm-pre-reloc;" properties to UART0, SPI
controller and SPI NOR flash node to enable usage in SPL. Otherwise
these devices will not be available.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dennis Gilmore <dgilmore@redhat.com>
Tested-by: Dennis Gilmore <dgilmore@redhat.com>